This invention relates to an electron emission device with an improved emission-widening characteristic, and relates to a method of driving the electron emission device.
Electron emission devices have functions of capturing electrons emitted from emitter by anode electrode and recognizing it as electrical signal, or enabling fluorescent part on anode electrode to emit light by electronic excitation in capturing electrons and recognizing it as optical signal. Application examples to recognize the electrical signal are amplifiers generally called vacuum tube, oscillators etc. On the other hand, application examples to recognize the optical signal are cathode-ray tubes, fluorescent display tubes, a flat-type display called FED (field emission display) etc.
As an example of the conventional electron emission devices, FED is explained below.
FIG. 1 is a cross sectional view illustrating one picture element in a conventional FED. Shown is a structure that electrons are emitted to excite red fluorescent part to emit light. Here, a picture element means a minimum unit when an image displayed by the FED is space-divided. In FED that a color picture is displayed by a display system of optical three primary colors, i.e. R(red), G(green) and B(blue), one of the colors, e.g. R(red), is called a picture element.
As shown in FIG. 1, SiO2 film of about 1 xcexcm thick, as insulation, film 2, is deposited on substrate 1 by sputtering, aluminum film of about 200 nm thick, as gate electrode 3, is deposited on the insulation film 2, a tubular gate hole 4 is formed penetrating the gate electrode 3 and insulation film 2. Emitter 5 is formed with cathode material deposited on the substrate 1 at the bottom of the gate hole 4. Also, anode electrode 6 is disposed around 5 mm above the substrate 1. Fluorescent material 7 with red fluorescent property is coated on part of the anode electrode 6 located just over the gate hole 4.
A voltage of about 5.1 kV is applied to the anode electrode 6 and the fluorescent part 7.0 V is applied to the emitter 5 of cathode material, and about 100 V is applied to the gate electrode 3. By thus applying the voltages, equipotential surface 8 is formed. Here, the distance, between the anode electrode 6 and the gate electrode 3 is 5 mm, and the voltage is 5000 V. So, electric field between the both electrodes 6 and 3 is given by:
5000/5 [V/mm]=1 [kV/mm].
On the other hand, the distance between the gate electrode 3 and the emitter 5 is 1 xcexcm (10xe2x88x923 [mm]), and the voltage is 100 V. So, the electric field between the both electrodes 3 and 5, gate and emitter is given by:
100/10xe2x88x923 [V/mm]=100 [kV/mm].
Thus, the value of electric field between gate and emitter is 100 timer, the value of electric field between the anode electrode 6 and the gate electrode 3. Therefore, at the inside of the gate hole 4 and its vicinity, the center of the gate hole 4 floats on the equipotential surface 8 to function as a circular hole lens. In this circular hole lens, in the vicinity of the center axis of the gate hole 4, electric field applied to the emitter 5 is weakened. In other words, the circular halt lens has power to diverge electrons, and electrons being emitted from positions off the center axis (convergence axis) are thus provided with electron tracks being curved in the direction of getting away from the axis (in the widening direction) Here, xe2x80x9cpowerxe2x80x9d of the circular hole lens means energy, which is owned by the equipotential surface 8 composing the circular hole lens, to diverge or converge electrons.
The phenomenon that the electron emission tracks are curved by the circular hole lens effect is detailed below. In FIG. 1 the emitter 5 is divided into small regions, and electron tracks emitted from the respective regions are shown. Since the gate hole 4 has an axially symmetrical shape, the emitter 5 deposited at the bottom of the gate hole 4 is divided into three regions from the near side of axis to the far side of axis. The regions sectioned by concentric circles like an archery target are called a, b and c from the center. Region a is a region that includes the axis. 9 is electron emitted from region a, 10 is electron emitted from region b, and 11 is electron emitted from region c. The respective electrons 9, 10 and 11 are widened outside in the order of regions a, b and c.
However, in the above electron emission device, there is a problem that electrons 9, 10 an 11 omitted are diverged in the direction of getting away from the axis while they travel to the anode electrode 6. Since in FED the picture elements are arrayed being divided finely, when elections are diverged, there occurs a problem that electron arrives at fluorescent part 7 of the neighboring picture element, not fluorescent part 7 of its own picture element. If electron arrives at fluorescent part 7 of a different color""s picture element, then a different color is created. Also, if electron arrives at fluorescent part 7 of a neighboring same-color""s picture element, then a failure in space revolution occurs.
FIG. 2 is a graph shaving an operation characteristic of the conventional electron emission device in FIG. 1. Shown in FIG. 2 is the relationship between cathode applied electric field and amount of emission current. The cathode applied electric field shown in the graph is applied to the emitter 5 deposited at the bottom of the gate hole 4. Herein, the value of electric field at the mean height (in 1 xcexcm insulation film structure, a height of 0.5 xcexcm) of gate hole is used as a value of cathode applied electric field.
When the cathode applied electric field increases gradually, electric field (threshold electric field) where emission current starts flowing appears. A maximum value in a necessary amount of emission current is called a maximum amount of current, and cathode applied electric field required to get that amount of current is called maximum electric field. To display an image in analogue system by FED, according to input signal, it is necessary to apply arbitrary electric field between the threshold electric field and the maximum electric field to get a desired brightness of fluorescent light. In pulse width drive system, a desired brightness of fluorescent light can be obtained by controlling the time of applying a certain electric field e.g., maximum electric field.
If there is a limit to the emission(or emission electron)-widening characteristic, the electron emission device only has to be driven according to the emission characteristic in FIG. 2. However, in fact, the emission may be, widened greater than the limit due to the circular hole lens effect. The bigger the difference of electric field at the opening of gate hole (phenomenon (1)) or the farther, the emission position of electron deviated from the axis (phenomenon (2)), the more significantly the widening phenomenon of electron due to the circular hole lens effect occurs.
FIG. 3 is a graph showing the phenomenon (1) when the difference of electric field at the opening of gate hole is big when the cathode applied electric field is increased while keeping the voltage of the anode electrode 6 constant, there occur phenomena such as (1xe2x80x2) the distortion of equipotential surface increases and (2xe2x80x2) the velocity at the opening of gate hole increases. By the phenomena (1xe2x80x2) and (2xe2x80x2)) the range of emission widening when arriving at the anode electrode 6 is further extended.
As xe2x80x9clength Dxe2x80x9d to represent the widening range above-mentioned in FIG. 3, the, range indicated by arrow D in FIG. 1 is used. In order to increase the cathode applied electric field, the potential of gate electrode is increased. That there occurs the problem that electron plunges into the neighboring fluorescent part means the widening range xe2x80x9cDxe2x80x9d is already exceeding the limit. Strictly thinking, since electron plunging into an invalid region such as a black matrix does not excite the fluorescent part, that there is electron not plunging into predetermined fluorescent part means the limit of widening is exceeded already. Although the critical widening varies as the case may be, it is definite that there is a certain limit. The widening range xe2x80x9cDxe2x80x9d is called critical widening length, and the cathode applied electric field to yield the characteristic is called critical electric field.
In electron emission devices used as FED or another application, to use cathode applied electric field greater than the critical electric field is allowed. So, the maximum electric field needed to get the maximum amount of current must be less than the critical electric field. The conventional electron emission devices using the gate hole structure are always subject to the influence of the circular hole lens effect since they are operated so that the inside electric field of the gate hole is more intensive than the outside one. Especially, the tendency that the degree of widening increases as it approaches the maximum amount of current is not desirable. In this tendency, even when a small ratio of emission electrons are plunging into a substandard region, due to the big total number of electrons, the influence is sufficiently remarkable. Alternatively, unnegligible number of electrons, as a noise, plunges into the substandard region.
Accordingly, it is an object of the invention to provide an electron emission device with emission-widening characteristics.
It is a further object of the invention to provide a method for driving such an electron emission device.
According to the invention, an electron emission device, comprises:
a substrate;
a gate electrode that has an opening and is disposed on the substrate;
an emitter that is formed in the opening; and
an anode electrode that is disposed at a predetermined interval from the emitter;
wherein a convergence electric field by which electrons to be emitted from the emitter is converged toward the anode electrode side is formed,
the convergence electric field has equipotential surface whose inclination increases according as it approaches the emitter, and
a relationship represented by expression below is satisfied:
{t(gk)/t(ak)}xc2x7Va less than Vg less than [{(t(g)+t(gk)}/t(ak)]xc2x7Va.
where distance between the surface of the emitter and the back surface of the gate electrode is t(gk), distance between the back surface of the anode electrode and the surface of the emitter in t(ak), potential of the anode electrode is Va, potential of the gate electrode is Vg, and thickness of the gate electrode is t(g).
In the electron emission device of this invention, the convergence electric field can function as a lens (circular hole lens) to converge electrons emitted from the cathode electrode (emitter), therefore, suppressing the emission widening not to widen the emission track of electron, keeping the arrival state of electron at the anode electrode properly. When electrons curved by the convergence of lens come to the over-focused state to cross electrons before the anode electrode, electrons arrive at the anode electrode in a widened distribution. However, the widening characteristic by the over-focused state is remarkably narrower than the case of the conventional electron emission devices which incur the effect of a divergence system lens by distortion of equipotential surface. The reason is as follows. Namely, when electric field given between gate electrode and emitter is smaller than electric field given between anode electrode and gate electrode, the electric field and the amount of electron emission are most around the center axis of the lens and the amount of electron emission reduces as it departs from the axis. By the influence of spherical aberration of lens, there occurs an electron emission distribution that at the further end of lens (position deviated from the axis) the bigger the divergence power is, and the amount of electron emission in most at the center of the axis and reduces as it departs from the center of the axis. Therefore, the electron-widening phenomenon can be prevented. On the other hand, in the divergence system lens of the conventional electron emission devices, electric field to effect the emitter around the center axis of the lens is weakest, the number of electrons traveling at the circumference are most, therefore accelerating the widening of electron. Comparing the widening phenomenon by over-focusing in this invention with the widening phenomenon by divergence system lens in the conventional electron emission devices, it is obvious that the widening of the over-focused system is smaller, when using the lens with similar power.
Here, it is preferable that the convergence electric field is formed by setting so that gate/emitter electric field given between the gate electrode and the emitter is smaller than gate/anode electric field given between the gate electrode and the anode electrode. In this case, electric field with the convergence effect of electron can be obtained easily.
Also, it is preferable that adding to the relationship between the gate/emitter electric field and the gate/anode electric field, even when the gate/emitter electric field is greater than the gate/anode electric field, the convergence electric field has equipotential surface whose. Inclination increases according as it approaches the emitter.
In this case, desired widening characteristic conditions can be used to the fullest. Provided that electrons emitted from the emitter have no lateral velocity or negligibly small originally, when there exists no lens effect by the distortion of equipotential surface, the linearity in movement of emission electron is best and the widening of electron arriving at the anode electrode is least. The state that no lens affect exists means that there is no change in electric field around the gate electrode. This state can be created by equalizing the gate/emitter electric field to the gate/anode electric field. When setting gate/emitter electric field greater than this state, the divergence system lens occurs slightly, the amount of electron emission also increases with an increase in electric field applied to the emitter. Even in this state with the slightly-occurring divergence effect, the widening characteristic may fall within the acceptable range, so, by operating the electron emission device including this range, the more amount of electron emission from the emitter can be captured.
Also, it is preferable that the emitter is formed as such a structure with a cone-shaped hollow that it protrudes at the sidewall of the opening toward the gate electrode. In this case, the convergence electric field can be suitably by the structure with a cone-shaped hollow.
Also, it is preferable that the emitter is formed narrower than the diameter of the opening. Alternatively, the emitter may be formed like a ring. Limited by that electrons emitted from the cathode electrode in the opening deviate from a desired range of tracks to the anode electrode, electric field more than that value may not be applied. However, in this composition, the emitter at the sidewall or center of the opening is removed. So, the limitation above can be eliminated and the higher electric field can be applied to the bottom of the opening.
Further, it is preferable that the emitter is of cathode materials that have emission characteristics different from each other. In this case, for example, by mixing cathode material of normal emission characteristics with cathode material of emission characteristics that, owing to high threshold electric field in electron emission, have a low degree of electron emission even when electric field more than the threshold electric field is applied, the range of selection for a desired emission and widening characteristic can be widened, as compared with using one kind of cathode material.
Also, it is preferable that a plurality of the openings are arrayed in the gate electrode, and the emitters formed in the openings are aligned each other. Thereby, the phenomenon that emission is widened uselessly in the lateral direction can be prevented.
Also, it is preferable that the opening is formed as a nearly rectangle, and the emitter is formed like a strip along the longitudinal direction of the opening. In this case, the corresponding openings to respective picture elements can be adjacent to each other, and the corresponding fluorescent parts to respective picture elements can be separated from each other in the direction orthogonal to the longitudinal direction of the opening.
Also, it is preferable that three picture elements, each of which is composed of the emitter and the gate electrode, are arrayed adjacent to each other,
the emitter of the first picture element located at the center of the three picture elements extends longitudinally at the center of the nearly rectangular opening,
the emitter of the second picture element located at one end of the three picture elements extends longitudinally on the first picture element side of the nearly rectangular opening, and
the emitter of the third picture element located at another end of the three picture elements-extends longitudinally on the first picture element side of the nearly rectangular opening.
In this case, for the composition that the corresponding fluorescent parts to respective picture elements are separated from each other, for example, the emission widening characteristic toward directly above is assigned to electrons emitted from the emitter of green picture element, the emission widening characteristic toward above left is assigned to electrons emitted from the emitter of red picture element, and the emission widening g characteristic toward above right is assigned to electrons emitted from the emitter of blue picture element.
Also, it is preferable that the gate electrode is formed as a nearly rectangle and a plurality of the openings are formed at the center and corners of the gate electrode, in the opening located at the center, the emitter extending longitudinal along the gate electrode at the center of the opening, in the openings located at the corners, the emitters extending shifted outside from the center of the openings. In this ease, the emission widening characteristics can be set suitably.
According to another aspect of the invention, an electron emission device, comprises:
a substrate;
a gate electrode that has an opening and is disposed on the substrate;
an emitter that is formed in the opening; and
an anode electrode that is disposed at a predetermined interval from the emitter;
wherein a convergence electric field by which electrons to be emitted from the emitter is converged toward the anode electrode side is formed,
the convergence electric field has equipotential surface formed like a leas with a convergence axis, and
the lens-like equipotential surface enables electrons to be emitted from the emitter to travel up to the anode electrode while being controlled from the just-focused state lying on the convergence axis to the over-focused state crossing the convergence axis.
Here, it is preferable that the electrons in the over-focused state shoot the anode electrode in a range narrower than the opening. In this case, emission electrons can be shot to the anode electrode focusing in a narrower range than the area of gate opening. When fluorescent material is coated on the anode electrode so that it has the same or larger size than the area of gate opening, all electrons emitted toward the anode electrode con be supplied to the fluorescent part.
According to another aspect of the invention, a method for driving the electron emission device composed as described above, wherein:
electric field between the anode electrode and the emitter is set as a threshold value.
In this driving method, it can be operated using a positive voltage by which the gate modulation potential is not biased. In using a display composed of multiple pixels, when the threshold electric fields of pixel are different each other, it is set to be a threshold value at the minimum potential. Some dispersion can be stored into external semiconductor storage and then used for the gate modulation.
According to another aspect of the invention, a method for driving the electron emission device composed as described above, wherein:
electric field between the anode electrode and the emitter is set as intermediate electric field between threshold electric field and electric field for maximum amount of current,
In this driving method, the electric field between anode and cathode in set to be electric field applied most frequently, and the gate potential is modulated in both polarities of positive/negative. Thereby, time when the gate potential is at zero can be elongated, and the drive consumption power can be reduced. Also, the electric field between anode and cathode is set to be the intermediate electric field between threshold electric field and electric field for maximum amount of current. Thereby, the amplitude of gate, potential modulation can be minimized.
According to another aspect of the invention, a method for driving the electron emission device composed as described above, wherein:
electric field between the anode electrode and the emitter is set as electric field for maximum amount of current.
In this driving method, the gate modulation can be performed by only negative potential., therefore the composition of gate modulation circuit can be simplified. Also, when only zero potential occurs due to a failure in the gate modulation circuit, or when only floating potential occurs due to breaking of wire, the screen takes the maximum brightness, therefore the failures can be checked quickly. So, it can be utilized as temporary emergency lighting.